The biosynthesis of ACTH, endorphin, Alpha-MSH, vasopressin and oxytocin, was studied, with emphasis on the enzymes involved in the proteolytic processing of the respective prohormones. A prohormone converting enzyme (PCE) which specifically cleaves between the Lys and Arg of Lys-Arg pairs of pro-opiomelanocortin (ACTH/endorphin prohormone) to form the active hormones, has been isolated from bovine pituitary intermediate and neural lobe secretory vesicles. PCE has now been purified to apparent homogeneity and characterized as a 68,000 molecular weight glycoprotein. A carboxypeptidase B-like enzyme and an aminopeptidase which function to remove the basic residues from the C- and N-terminals respectively, from the peptide hormone, following the action of PCE, have been detected in intermediate and neural lobe secretory vesicles. The regulation of biosynthesis of pro-opiomelanocortin in the toad intermediate lobe by dopamine and cyclic AMP was also studied.